This invention relates to an arrangement for limiting the swing angle of a boom for a rock drilling unit when the boom is being vertically lifted and lowered, wherein the rock drilling unit comprises a boom, pivotally connected, relative to a frame, about vertical and horizontal shafts, and a swing cylinder and a tilt cylinder between the frame and the boom, in whose cylinder spaces hydraulic fluid may be fed for turning the boom relative to the frame.
A problem with booms for rock drilling units is that the swing angle of a boom changes when the angle of elevation of the boom changes, causing significant widening of the swing angle of the boom at its extreme height values and excessive lateral movement of the boom. Consequently, in known solutions, the swing angles of a boom are defined narrow enough for preventing excessive turning of the boom. This again results in the operating range of a boom being in some cases far too narrow as the whole available width area cannot be utilized in the mid-part of the boom because of problems with the upper and lower angles.
The object of the present invention is to provide an arrangement for eliminating the drawbacks of known solutions and allowing optimal utilization of the operating range of a boom. The arrangement of the invention is characterized in that it comprises control means for indicating the widest permissible swing angle of a boom, that the control means are coupled to control a swing cylinder so that if the boom, when being lifted or lowered, tends to turn outside the widest permissible swing angle, the control means let hydraulic fluid flow into the cylinder spaces of the swing cylinder for turning the boom to a reverse direction with respect to the turning movement caused by the lifting or lowering movements, thereby preventing the boom from exceeding the widest permissible swing angle.
It is an essential idea of the invention that the control means are coupled to the boom movably therewith and a relief valve is coupled to be controlled by the control means, whereat, with the boom turning to a predetermined swing angle, the control means control the relief valve so that it feeds hydraulic fluid into the swing cylinder of the boom and, correspondingly, discharges hydraulic fluid therefrom for maintaining the boom in this widest permissible position of the swing angle when it is lifted or lowered farther into the same direction, upward or downward, respectively.
It is an advantage of the invention that the widest possible operating range may be defined for a boom when it is essentially in the horizontal plane, whereat, with the arrangement of the invention, the same operating width is maintained for a boom both in its upper and lower positions without the joints or other parts of boom cylinders ending up in an unusual or operationally impossible position. This allows maximum boom coverage without problems of known solutions.